Study of environment
Classic formal genetics, i.e. family, twin, and adoption studies, demonstrated the genetic component of mental illness, with heritability estimates ranging between 40 % (major depression) and 80 % (bipolar disorder). One often forgets that they also tell us about a non-negligible environmental influence. Gene-environment interactions, e.g. for the serotonin-transporter length polymorphism or monoamine-oxidase, have been reported as important factors in the etiology of psychiatric disorders; however, there is still a lack of large-scale, systematic studies accounting robustly for these gene-environment interactions. Prospective studies will be key elements of success in this area, once genetic susceptibility factors of psychiatric disorders such as schizophrenia or bipolar disorder begin to emerge.